Bought a pair of Magnepan LRS. Should I buy a Schiit Vidar or Bryston 3B-ST?

Magnepan and bridged amps rarely a good match 

No experience with how they would sound with Magnepan, but used Parasound A21s have a lot of juice and are selling at a bargain since the new A21+ was introduced.

Schiit uses a pair of Vidar with 1.7s in their showroom so I'm not worried about stressing them out.

I have two Vidars in a secondary system and they have effortlessly driven everything I have thrown at them, including Maggie 1.7.  In mono, each Vidar delivers 400W into 8 ohm and even though it is not officially rated for 4 ohm, it will handle just about any 4ohm load. I spoke with Schiit and they told me the Vidar MIGHT go into protection with a 4ohm load if you are playing at extreme volumes.  Assuming you are not putting on a concert in your living room, you should be more than safe. If it does trigger though, the Vidar will go into protection mode and will not be damaged. The one idiosyncratic thing is the Vidar only takes balanced connected in mono and only takes SE connections is stereo. Good luck!

I own a pair of Magnepan 1.7 speakers. I started out using a pair of Schiit Vidars as monoblocks. For the most part they were fine. If you play classical or jazz at reasonable volumes you should be ok, and mine were ok. But if you play loud rock or fusion jazz you will not have enough headroom and one of my Vidars went into thermoprotection will playing Jeff Beck fusion loud. I sold the Vidars and got a pair of Bryston 4B3s. They are the ticket. Plenty of clean power. No protection issues. Plenty of headroom. Not sure about the 3B3s. But you can find the Bryston 4B3 on the used market frequently at much reduced below MSRP. I would still be using my 4B3s if it not for the amazing deal I got a while ago on a pair of 28B3s. Needless to say, I am set power amp-wise. I’m done. Bottomline: if you want headroom go with Bryston. If you are never going to play your system loud get a pair of Vidars (not one).